Method and system for fabricating an optical fiber device for shape sensing

ABSTRACT

There is described a method of fabricating an optical fiber device, the method comprising: positioning longitudinal portions of a plurality of optical fibers alongside each other in a given geometrical relationship, depositing liquid coating material around the longitudinal portions of the plurality of optical fibers; and the liquid coating material setting up around the longitudinal portions of the plurality of optical fibers thereby maintaining said given geometrical relationship along the longitudinal portions.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority on U.S. Patent Application No.62/751,951 filed Oct. 29, 2018, the entire contents of which areincorporated herein by reference.

FIELD

The improvements generally relate to fabrication of optical fiberdevices, and more particularly relate to fabrication of optical fiberdevices for two- or three-dimensional shape sensing purposes.

BACKGROUND

Two- or three-dimensional shape sensors generally involve an opticalfiber device having two or more optical fibers extending alongside eachother in a given geometrical relationship, an interrogator opticallycoupled to the optical fibers, and a computing device communicativelycoupled to the interrogator. As the optical fiber device experiences acurvature-induced strain along its length, a relative longitudinaltension or compression with respect to the center of the optical fiberdevice will cause the optical fibers to register positive- ornegative-induced strain changes, respectively. To calculate a localcurvature, the relative strains are measured in real-time by theinterrogator and the measured strains are processed by the computingdevice according to known algorithms. Although optical fiber devices forshape sensing applications are satisfactory to a certain degree, thereremains room for improvement, especially in providing fabricationmethods and systems which can limit variations in the positioning of theoptical fibers with respect to one another along the length of theoptical fiber device.

SUMMARY

In accordance with a first aspect of the present disclosure, there isprovided a method of fabricating an optical fiber device, the methodcomprising: positioning longitudinal portions of a plurality of opticalfibers alongside each other in a given geometrical relationship,depositing liquid coating material around the longitudinal portions ofthe plurality of optical fibers; and the liquid coating material settingup around the longitudinal portions of the plurality of optical fibersthereby maintaining said given geometrical relationship along thelongitudinal portions.

Further in accordance with the first aspect of the present disclosure,said depositing can for example include extruding the liquid coatingmaterial around the longitudinal portions of the plurality of opticalfibers while maintaining said given geometrical relationship.

Still further in accordance with the first aspect of the presentdisclosure, said extruding can for example include forcing the liquidcoating material at a longitudinal position around the longitudinalportions of the plurality of optical fibers and moving the longitudinalportions of the plurality of optical fibers longitudinally during saidforcing.

Still further in accordance with the first aspect of the presentdisclosure, said moving can for example include longitudinally pullingon ends of the plurality of optical fibers.

Still further in accordance with the first aspect of the presentdisclosure, said moving can for example be performed as the longitudinalportions of the plurality of optical fibers are longitudinally receivedin an opening having an inner surface confining the plurality of opticalfibers into the given geometrical relationship.

Still further in accordance with the first aspect of the presentdisclosure, said depositing can for example include maintaining theplurality of optical fibers parallel to one another.

Still further in accordance with the first aspect of the presentdisclosure, said depositing can for example include melting coatingmaterial thereby forming the liquid coating material, and wherein saidsetting up includes cooling the liquid coating material.

Still further in accordance with the first aspect of the presentdisclosure, the method can for example comprise, prior to saiddepositing, heating a plurality of optical fiber preforms, and drawingthe plurality of optical fiber preform into the plurality of opticalfibers.

Still further in accordance with the first aspect of the presentdisclosure, the method can for example comprise, after said drawing,inscribing one or more optical gratings along a portion of each of theplurality of optical fibers.

Still further in accordance with the first aspect of the presentdisclosure, at least one of the optical gratings can for example have arandom continuous distribution such that a return signal propagating insaid optical fiber has a full width at half maximum bandwidth rangingbetween about 0.1 THz and about 40 THz.

In accordance with a second aspect of the present disclosure, there isprovided a system for fabricating an optical fiber device, the systemcomprising: a coating material source having liquid coating material; adie having a longitudinal conduit receiving a plurality of opticalfibers extending alongside each other; an optical fiber positionerconfining longitudinal portions of the plurality of optical fibers in agiven geometrical relationship relative to one another; and a coatingdevice in fluid communication with the coating material source and withthe longitudinal conduit of the die, the coating device flowing theliquid coating material around at least the longitudinal portions of theplurality of optical fibers when received in the longitudinal conduitand positioned in the given geometrical relationship, the coatingmaterial setting up around the longitudinal portions of the plurality ofoptical fibers thereby maintaining said given geometrical relationshipalong the longitudinal portions.

Further in accordance with the second aspect of the present disclosure,the optical fiber positioner can for example be within the longitudinalconduit of the die.

Still further in accordance with the second aspect of the presentdisclosure, the optical fiber positioner can for example include anozzle an opening with an inner surface of a given shape, the innersurface confining the longitudinal portions of the plurality of opticalfibers in the given geometrical relationship, and an outer surface uponwhich the liquid coating material flows.

Still further in accordance with the second aspect of the presentdisclosure, the inner surface of the opening of the nozzle can forexample have a dimension below 1 mm, preferably below 500 μm and morepreferably below 400 μm.

Still further in accordance with the second aspect of the presentdisclosure, the system can for example comprise a pulling mechanismpulling on ends of the plurality of optical fibers in a longitudinalorientation as the liquid coating material is flowed around theplurality of optical fibers.

Still further in accordance with the second aspect of the presentdisclosure, the system can for example comprise an inscription deviceupstream from said flow mechanism, the inscription device inscribing oneor more optical gratings along a portion of each of the plurality ofoptical fibers.

Still further in accordance with the second aspect of the presentdisclosure, the system can for example have a heater melting solidcoating material to obtain the liquid coating material. The heater maybe part of the coating material source.

Still further in accordance with the second aspect of the presentdisclosure, the system can for example have a cooler for cooling themelted liquid coating material after deposition. The cooler may be partof the coating material source or be a standalone component in thermalcommunication with at least a portion of the die.

Still further in accordance with the second aspect of the presentdisclosure, the cooler can be at least one of a fluid flow cooler suchas a forced air flow cooler, a water-based cooler and the like, and/or athermoelectric unit such as a Peltier module.

Still further in accordance with the second aspect of the presentdisclosure, said positioning can for example include positioning atleast an additional component relative to the plurality of opticalfibers, said depositing including depositing liquid coating materialalso around said additional component; the liquid coating materialsetting up around the additional component as well. The additionalcomponent can be one or more of any one of the following group ofcomponents: an electrical wire, a conductive glass fiber, a capillaryfiber, a photonic crystal fiber, a laser delivery fiber and any othersuitable component.

In accordance with a third aspect of the present disclosure, there isprovided an optical fiber device having a plurality of optical fiberseach having a respective longitudinal portion extending alongside eachother in a given geometrical relationship, a coating layer around thelongitudinal portions of the plurality of optical fibers, the coatinglayer maintaining the plurality of optical fibers in the givengeometrical relationship along the longitudinal portions of theplurality of optical fibers.

Further in accordance with the third aspect of the present disclosure,the given geometrical relationship can for example be a triangle.

Still further in accordance with the third aspect of the presentdisclosure, the triangle can for example be an isosceles triangle.

Still further in accordance with the third aspect of the presentdisclosure, the triangle can for example be an equilateral triangle,with the longitudinal portions of the plurality of optical fibers beingadjoining to one another.

Still further in accordance with the third aspect of the presentdisclosure, the optical fiber device can for example comprise one ormore optical gratings along said longitudinal portions of each of theplurality of optical fibers.

Still further in accordance with the third aspect of the presentdisclosure, at least one of the optical gratings can for example have arandom continuous distribution such that a return signal propagating insaid optical fiber has a full width at half maximum bandwidth rangingbetween about 0.1 THz and about 40 THz.

Still further in accordance with the third aspect of the presentdisclosure, the optical fiber device can for example include at least anadditional component relative to the plurality of optical fibers insidesaid coating layer. The additional component can be one or more of anyone of the following group of components: an electrical wire, aconductive glass fiber, a capillary fiber, a photonic crystal fiber, alaser delivery fiber and any other suitable component.

In accordance with a fourth embodiment of the present disclosure, thereis provided a distributed temperature and strain sensing (DTSS) systemcomprising: an optical interrogator; an optical coupler assembly havingan input being optically coupled to the optical interrogator and aplurality of outputs; and a plurality of optical fiber devices having atleast: a first optical fiber device having a first sensing optical fiberbeing serially connected to a first one of the plurality of outputs ofthe optical coupler assembly; and a second optical fiber device havingan optical path extender being serially connected to a second one of theplurality of outputs of the optical coupler assembly, the optical pathextender having an optical path length being equal to or greater than anoptical path length of the first optical fiber device, and a secondsensing optical fiber being serially connected to the optical pathextender; wherein, during use, the optical interrogator is configuredfor emitting an optical signal at the input of the optical couplerassembly, and for receiving, in response to said emitting, a firstreturn optical signal returning from the first sensing optical fiber anda second return optical signal returning from the second sensing opticalfiber, the first and second return optical signal being temporallydelayed from one another due to the difference in their correspondingoptical path lengths.

Further in accordance with the fourth embodiment of the presentdisclosure, the DTSS system can comprise for example a third opticalfiber device having an optical path extender being serially connected toa third one of the plurality of outputs of the optical coupler assembly,the optical path extender of the third optical fiber device having anoptical path length being equal to or greater than an optical pathlength of the second optical fiber device, and a third sensing opticalfiber being serially connected to the optical path extender of the thirdoptical fiber device; wherein, during use, the optical interrogator isconfigured for receiving a third return signal returning from the thirdsensing fiber, the first, second and third return signal beingtemporally delayed from one another.

Still further in accordance with the fourth embodiment of the presentdisclosure, the DTSS system can for example comprise a fourth opticalfiber device having an optical path extender being serially connected toa fourth one of the plurality of outputs of the optical couplerassembly, the optical path extender of the fourth optical fiber devicehaving an optical path length being equal to or greater than an opticalpath length of the third optical fiber device, and a fourth sensingoptical fiber being serially connected to the optical path extender ofthe fourth optical fiber device; wherein, during use, the opticalinterrogator is configured for receiving a fourth return signalreturning from the fourth sensing fiber, the first, second, third andfourth return signal being temporally delayed from one another.

Still further in accordance with the fourth embodiment of the presentdisclosure, at least one of the first and second sensing optical fiberscan for example have a scatter increasing device being configured forincreasing scattering of the corresponding optical signal as itpropagates along the at least one of the first and second sensingoptical fibers.

Still further in accordance with the fourth embodiment of the presentdisclosure, the scatter increasing device can for example be an opticalgrating inscribed along a portion of a corresponding one of the firstand second sensing optical fibers, the optical grating having a randomcontinuous distribution such that a return signal, caused by propagationof an optical signal therealong, has a full width at half maximumbandwidth ranging between about 0.1 THz and about 40 THz.

Still further in accordance with the fourth embodiment of the presentdisclosure, said full width at half maximum bandwidth can for examplerange between about 0.35 THz and about 7 THz.

Still further in accordance with the fourth embodiment of the presentdisclosure, the grating can for example have a coherence length rangingbetween about 2λ and about 500λ when the return signal has a scatteringspectrum with a Gaussian shape, wherein λ denotes a wavelength of theoptical signal.

Still further in accordance with the fourth embodiment of the presentdisclosure, the coherence length can for example range between about 10λand about 100λ.

Still further in accordance with the fourth embodiment of the presentdisclosure, the optical coupler assembly can for example be a one-by-twofiber coupler.

Still further in accordance with the fourth embodiment of the presentdisclosure, the optical coupler assembly can for example have one ormore a X-by-Y fiber couplers, X and Y being positive integers.

Still further in accordance with the fourth embodiment of the presentdisclosure, the optical coupler assembly can for example have aplurality of two-by-two fiber couplers being connected to one another.

In accordance with a fifth aspect of the present disclosure, there isprovided an optical device comprising: a length of optical fiberconfigured for propagating an optical signal; and an optical gratinginscribed along a portion of the length of optical fiber, the opticalgrating having a random continuous distribution such that a returnsignal caused by said propagating has a full width at half maximumbandwidth ranging between about 0.1 THz and about 40 THz.

Further in accordance with the fifth aspect of the present disclosure,said full width at half maximum bandwidth can for example range betweenabout 0.35 THz and about 7 THz.

Still further in accordance with the fifth aspect of the presentdisclosure, the grating can for example have a coherence length rangingbetween about 2λ and about 500λ when the return signal has a scatteringspectrum with a Gaussian shape, wherein λ denotes a wavelength of theoptical signal.

Still further in accordance with the fifth aspect of the presentdisclosure, the coherence length can for example range between about 10λand about 100λ.

Still further in accordance with the fifth aspect of the presentdisclosure, the random continuous distribution can for example be arandom phase distribution.

Still further in accordance with the fifth aspect of the presentdisclosure, the random continuous distribution can for example be arandom period distribution.

Still further in accordance with the fifth aspect of the presentdisclosure, the random continuous distribution can for example be arandom amplitude distribution.

Still further in accordance with the fifth aspect of the presentdisclosure, the random continuous distribution can for example be one ormore from the group consisting of: a random phase distribution, a randomperiod distribution and a random amplitude distribution.

Still further in accordance with the fifth aspect of the presentdisclosure, the optical grating can for example have a length being atleast in the centimeter range.

Many further features and combinations thereof concerning the presentimprovements will appear to those skilled in the art following a readingof the instant disclosure.

DESCRIPTION OF THE FIGURES

In the figures,

FIG. 1 is a schematic view of an example of a shape sensing devicehaving an optical fiber device, in accordance with one or moreembodiments;

FIG. 1A is a graph representing a three-dimensional model of the opticalfiber device of FIG. 1, in accordance with one or more embodiments;

FIG. 2 is an enlarged view of a portion of the optical fiber device ofFIG. 1, in accordance with one or more embodiments;

FIG. 3 is a flow chart of an example method of fabricating an opticalfiber device, in accordance with one or more embodiments;

FIG. 4A is an oblique view of three optical fibers extending alongsideeach other in a given geometrical relationship, in accordance with oneor more embodiments;

FIG. 4B is an oblique view of the three optical fibers of FIG. 4A havingliquid coating material around longitudinal portions of the threeoptical fibers, in accordance with one or more embodiments;

FIG. 4C is an oblique view of an optical fiber device having the threeoptical fibers of FIG. 4A, and a coating layer being formed as theliquid coating material of FIG. 4B sets up, in accordance with one ormore embodiments;

FIG. 5 is a schematic view of an example of a system for fabricating anoptical fiber device, in accordance with one or more embodiments;

FIG. 6 is a schematic view of the system of FIG. 5, showing a draw towerand an inscription device, in accordance with one or more embodiments;

FIG. 7A is a graph showing return loss as function of wavelength, asmeasured using an example optical fiber device, in accordance with oneor more embodiments;

FIG. 7B is a graph showing amplitude as function of length of theexample optical fiber device of FIG. 7A, in accordance with one or moreembodiments;

FIG. 8A is a cross sectional view of the example optical fiber device ofFIG. 7A, in accordance with one or more embodiments;

FIG. 8B is a top plan view of the example optical fiber device of FIG.7A, in accordance with one or more embodiments;

FIG. 9 is an oblique view of another example of an optical fiber device,in accordance with one or more embodiments;

FIG. 9A includes cross-sectional and top plan views of the optical fiberdevice of FIG. 9, in accordance with one or more embodiments;

FIG. 9B is a cross-sectional view of the optical fiber device of FIG. 9,with an optical fiber triplet maintained in an exemplary geometricalrelationship, in accordance with one or more embodiments;

FIG. 10A is a graph showing radius of the optical fiber triplet of FIG.9B as function of a length of the optical fiber device of FIG. 9, inaccordance with one or more embodiments;

FIG. 10B is a graph showing angles of the optical fiber triplet of FIG.9B as function of a length of the optical fiber device of FIG. 9, inaccordance with one or more embodiments;

FIG. 11 is a schematic view of an example of a DTSS system, inaccordance with the prior art;

FIG. 12 is a schematic view of an example of a DTSS system, inaccordance with one or more embodiments;

FIG. 12A is a graph showing exemplary data produced by the DTSS systemof FIG. 12, in accordance with one or more embodiments;

FIG. 13 is a schematic view of a computing device of the controller ofthe DTSS system of FIG. 12, in accordance with one or more embodiments;

FIG. 14 is a schematic view of another example of a DTSS system, withthree sensing optical fibers, in accordance with one or moreembodiments;

FIG. 15 is a schematic view of another example of a DTSS system, withfour sensing optical fibers, in accordance with one or more embodiments;

FIG. 16 is an enlarged view of a scatter increasing device of the DTSSsystem of FIG. 12, in accordance with one or more embodiments;

FIG. 17A is a schematic view of another example of a DTSS system, with aLUNA system configured for taking measurements (e.g., a 42-nm wide scan)in a sensing optical fiber, in accordance with one or more embodiments;

FIG. 17B is a graph showing return optical signal as function ofposition as taken with the LUNA system of FIG. 17A for a sensing opticalfiber including i) a standard SMF-28 region, ii) a uniform fiber Bragggrating (FBG) region, iii) a random FBG and iv) continuous UV exposureregion with no FBG;

FIG. 17C is a graph showing return optical signal as function ofposition as taken with the LUNA system of FIG. 17A for a sensing opticalfiber including i) a standard SMF-28 region, ii) a UV-exposed portion ofa high numerical aperture/high Germanium-doped core (hereinafter“HNA/high Ge core”) optical fiber, and iii) a plain portion of theHNA/high Ge core optical fiber;

FIG. 18 is a graph showing collected scattering intensity as function ofpower at 213 nm for a sensing optical fiber, showing an increase incollected scatter intensity with UV exposure power at constant speed ofexposure;

FIG. 19 is a graph showing root mean square (RMS) noise level asfunction of a length for a UV-exposed SMF-28, an unexposed SMF-28, aUV-exposed HNA/High Ge core, and an unexposed HNA/High Ge core;

FIG. 20A is a graph showing temperature change as function of positionfor a sensing optical fiber maintained at constant temperature;

FIG. 20B is a graph showing temperature change as function of positionfor a sensing optical fiber being locally heated by a thin wire;

FIG. 21 is a schematic view of an example of a system for inscribing aFBG along an optical fiber using a phase mask and a Talbotinterferometer, in accordance with one or more embodiments;

FIG. 22 is a schematic view of an example of a system for inscribing aFBG along an optical fiber using a phase mask, in accordance with one ormore embodiments;

FIG. 23A is a graph showing backscattered intensity as function ofposition for a FBG inscribed in SMF-28 fiber using the system of FIG.21, with 37 mW of laser power, at a writing speed of 0.2 mm/s, themeasurements were taken with an 88.24 nm bandwidth on an OBR 4600;

FIG. 23B is a graph showing return loss as function of wavelength forthe FBG of FIG. 23A;

FIG. 24 is a graph showing gain as function of writing speed for a FBGinscribed using the system of FIG. 21 in a SMF-28 fiber from Corning anda SM1500 fiber from FiberCore using an amplitude of 5 V and a frequencyof 20 Hz, with 22 mW and 37 mW of laser power;

FIG. 25 is a graph showing RMS as function of bandwidth for 80 cm ofunexposed SMF-28 fiber and for over 30 cm of a Random Optical Grating byUltraviolet Exposure fiber Bragg grating (hereinafter referred to as“ROGUE FBG”) inscribed using the system of FIG. 21 when maintained atconstant temperature and averaged over 15 measurements;

FIGS. 26A-F include graphs showing accuracy of both unexposed SMF-28fiber and FBG inscribed using the system of FIG. 21, as the 115 cm fiberis being stretched from 0 to 20 μm, in 1 μm increments, for all scanningbandwidths provided by the OBR, namely (a) 1.31 nm, (b) 2.61 nm, (c)5.24 nm, (d) 10.51 nm, (e) 21.16 nm, and (f) 42.90 nm;

FIG. 27 is a graph showing spectral shift RMS error as function ofbandwidth for both the unexposed fiber and a 10 cm FBG inscribed usingthe system of FIG. 21, calculated over the 20 μm stretching and acrossan 8 cm sensing region;

FIG. 28 is a graph showing spectral shift RMS error as function of gainfor a FBG inscribed using the system of FIG. 21, calculated over a 20 μmstretching and across an 8 cm sensing region;

FIG. 29 is a graph showing spectral shift RMS error as function of lossfor three FBGs of different strengths inscribed using the system of FIG.21, calculated over a 20 μm stretching and across 8 cm of fiber;

FIG. 30A is a graph showing full width at half maximum bandwidth of aFBG as function of length, as modeled for FBGs having lengths rangingbetween 1 mm and 10 m, shown on a logarithmic scale;

FIG. 30B is a graph showing full width at half maximum bandwidth of aFBG as function of length, including modeled and experimental data forFBGs having lengths ranging between 2 mm to 40 cm, shown on a linearscale; and

FIG. 31 is a graph showing RMS noise level as a function of gauge lengthranging between 0.5 mm and 20 cm, for both unexposed fiber and ROGUEFBG, when placed in an insulated box, averaged over 15 measurements, forFWHM bandwidths of 5.24 and 42.90 nm.

DETAILED DESCRIPTION

FIG. 1 shows an example of a shape sensing system 10, in accordance withan embodiment. As depicted, the shape sensing system 10 has an opticalfiber device 12, and an electro-optic module 14. The electro-opticmodule in this example has an optical interrogator 16 optically coupledto the optical fiber device 12 and a computing device 18 communicativelycoupled to the optical interrogator 16. The computing device 18 is shownas a part within the electro-optic module 14 in this example, but couldalso be remote therefrom.

As shown, the optical fiber device 12 has a body 20 and two, three ormore optical fibers 22 extending alongside each other within the body 20in a given geometrical relationship relative to one another. As shown,when so-positioned, the optical fibers 22 are radially spaced-apart froma center of the body 20 of the optical fiber device 12. Morespecifically, in this example, the optical fibers 22 arecircumferentially distributed around the center of the body 20 of theoptical fiber device 12.

In this embodiment, the optical interrogator 16 is configured fortransmitting optical signals along the optical fiber device 12 and forreceiving return optical signals from the optical fiber device 12.Further, the optical interrogator 16 is configured to transmit electricsignals to the computing device 18, the electric signals beingrepresentative of the received return optical signals. Based on thereceived electric signals, the computing device 18 is adapted andconfigured to generate a two- or three-dimensional model representingthe shape and orientation of the optical fiber device 12 at a specificmoment in time. For instance, FIG. 1A shows a plot of a model 24 in agiven coordinate system (x, y, z) generated by the computing device 18,which represents the shape and orientation of the optical fiber device12 shown in FIG. 1.

Accordingly, by monitoring the model of the optical fiber device 12 overtime, the shape sensing system 10 allows the monitoring of the shape andthe orientation of the optical fiber device 12 in real time or quasireal time. In some embodiments, the shape sensing system 10 generallyhas a small footprint and is lightweight, which can provide the abilityto track instruments, bones and/or limbs, with a millimeter-levelaccuracy in some embodiments. In this embodiment, the shape sensingsystem 10 has one optical fiber device, having for example a diameter of600 microns and a longitudinal length up to a few meters, at least 50 mor more. Alternately or additionally, the shape sensing system 10 canhave more than one optical fiber device, with different diameters and/ordifferent longitudinal lengths.

FIG. 2 shows a portion of the optical fiber device 12 of FIG. 1. Asdepicted, the optical fiber device 12 has three optical fibers 22 a, 22b and 22 c which extend along a longitudinal length l of the opticalfiber device 12. In some other embodiments, the optical fiber device 12can have two optical fibers for two-dimensional shape sensing, or atriplet of optical fibers (i.e. three optical fibers), four or moreoptical fibers for three-dimensional shape sensing.

In this example, the optical fibers 22 a, 22 b and 22 c are embedded inthe body 20. As will be described below, the body 20 is made of acoating material. Example of such coating material can include, but notlimited to, M6823MZ from Total and any other suitable coating material.Preferably, the coating material can be melted to a liquid form duringthe fabrication process, after which it can sets up to a given shapearound the optical fibers 22 a, 22 b and 22 c and maintain that shapeover time. As shown in this example, each of the optical fibers 22 a, 22b and 22 c has a respective core 26 surrounded by one or more innercladdings 28. The inner claddings 28 generally have a refractive indexwhich is lower than a refractive index of the cores 26 to allow opticalpropagation therein. The refractive index of the cores 26 and/or innercladdings 28 need not to be identical from one optical fiber to another.As shown in this example, the optical fiber device 12 can have a sheath30 covering the body 20 of the optical fiber device 12, for providing atleast some mechanical resistance and/or thermal insulation.

As depicted, the optical fibers 22 a, 22 b and 22 c are off-axis andcircumferentially spaced-apart from one another within the body 20. Inthis example, the optical fibers 22 a, 22 b and 22 c arecircumferentially spaced-apart by 60° and therefore are positioned in anequilateral triangle shape. In this specific example, the optical fibers22 a, 22 b and 22 c are sized and shaped to be single-mode for lighthaving a wavelength of 1550 nm. In alternate embodiments, however, eachoptical fiber may be sized and shaped to be multimode.

The shape sensing system 10 involves distributed strain measurements ineach of the optical fibers 22 a, 22 b and 22 c of the optical fiberdevice 12, at different longitudinal positions li along its longitudinallength l, to construct the model 24 discussed with reference to FIG. 1A.In this example, i is an integer ranging from 1 and a number N oflongitudinal positions. The longitudinal increment Δl between twosuccessive longitudinal positions li can be in the order of themillimeter for example. The longitudinal increments Δl betweensuccessive longitudinal positions li need not be identical for each pairof successive longitudinal positions li where strain measurements aretaken.

In the context of the optical fiber device 12, bending of the opticalfiber device 12 induces strain on each one of the optical fibers 22 a,22 b and 22 c, which can be measured by propagating light into each ofthe cores 26 during the bending and by monitoring reflected wavelengthsresulting from said propagation. The induced strains are a function ofthe local degree of bending of the optical fiber device 12. Forinstance, more strain is induced in the optical fiber device 12 aroundits elbow portion than in any of its straight portions. To measurestrain in a single optical fiber 22, light is sent down the core 26which causes light to be reflected at different longitudinal positionsalong the core 26. Wavelengths of the reflected light are a function ofthe strain on the core 26 and of its temperature. The amount ofreflected light can be enhanced by inscribing one or more fiber Bragggratings (FBGs) 23 along the cores 26 of the optical fibers, asillustrated. As shown in this example, the cores 26 of the opticalfibers 22 a and 22 b have discrete FBGs 23 whereas the core 26 of theoptical fiber 22 c has a continuous FBG 23′ inscribed along its length.It is intended that any one of the optical fibers 22 of the opticalfiber device 12 can have at least a discrete FBG and/or at least acontinuous FBG, depending on the embodiment. It is envisaged that usingcontinuous FBGs, such as the ROGUE FBG described in detail below, canprovide a continuous enhancement. Instead of using FBGs at discretelocations, the entire optical fiber's length can provide backscatteredsignal. During signal processing, using OFDR and/or OTDR sensingtechniques, sensing can thus be performed at a multitude of points alongthe fiber's length. The spacing between each of those sensing pointsdefines the spatial resolution. Contrarily to conventional FBG sensingschemes, those sensing points can be anywhere on the fiber, and thesensor spacing can be tuned, e.g., depending on signal acquisition andprocessing times, as well as scanned bandwidth. The shape sensingalgorithm afterwards may be similar as it not only depend on thepositioning of the discrete FBGs, but on the sensor spacing set by theinterrogator data processing. Otherwise, the reflected light can includeRayleigh scattering, for instance. To reduce undesirable effects oftemperature during the strain measurements, the sheath 30 of the opticalfiber device 12 can provide at least some thermal insulation.

The optical fibers 22 a, 22 b and 22 c allow at least two non-coplanarpairs of optical fibers to be formed. For instance, in this embodiment,the optical fibers 22 a and 22 b form a first pair of optical fiberslying in a first plane 34 a, and the optical fibers 22 a and 22 c form asecond pair of optical fibers lying in a second plane 34 b that is notcoplanar with the first plane 34 a. As having only the first pair ofoptical fibers would allow reconstruction of the bending of thecorresponding waveguide only in the first plane 34 a, having the twonon-coplanar pairs of optical fibers can allow reconstruction of thebending of the corresponding waveguide in both the first and secondplanes 34 a and 34 b, thus allowing a three dimensional model of theoptical fiber device 12 to be determined based on the known geometricalrelationship between the optical fibers 22 a, 22 b and 22 c.

It was found that limiting the variation in the positioning of theoptical fibers 22 a, 22 b and 22 c along a length of the optical fiberdevice 12 could result in more accurate shape sensing. Indeed, shouldthe geometrical relationship between the optical fibers 22 a, 22 b and22 c vary uncontrollably along the length of the optical fiber device12, undesirable biases could be introduced in the shape sensing. Thereis therefore described herein methods and systems for fabricating anoptical fiber device destined for shape sensing applications and anyother suitable sensing applications.

FIG. 3 shows an example method 300 of fabricating an optical fiberdevice. Description of the method 300 will be made with respect to theoptical fiber device 12 of FIG. 2 for ease of reading, with referencesto FIGS. 4A, 4B and 4C.

At step 308, longitudinal portions 36 a, 36 b and 36 c of the opticalfibers 22 a, 22 b and 22 c are positioned alongside each other in agiven geometrical relationship 38, an example of which is shown at FIG.4A.

At step 310, liquid coating material 40 is deposited around thelongitudinal portions 36 a, 36 b and 36 c of the optical fibers 22 a, 22b and 22 c, such as shown at FIG. 4B.

At step 312, and as illustrated at FIG. 4C, the liquid coating material40 sets up around the longitudinal portions 36 a, 36 b and 36 c of theoptical fibers 22 a, 22 b and 22 c, to form the body 20, therebymaintaining the given geometrical relationship 38 along the longitudinalportions 36 a, 36 b and 36 c.

As such, as long as the positioning of the optical fibers 22 a, 22 b and22 c relative to one another is maintained until the liquid coatingmaterial 40 set up, the given geometrical relationship 38 can beexpected to be maintained all along the longitudinal portions 36 a, 36 band 36 c of the optical fibers 22 a, 22 b and 22 c of the optical fiberdevice 12.

In some embodiments, the optical fibers 22 a, 22 b and 22 c have one ormore outer claddings or jackets which are removed prior to performingsteps 308, 310 and 312. However, in some other embodiments, the opticalfibers 22 a, 22 b and 22 c can also be freshly drawn optical fibers. Forinstance, in such embodiments, the method 300 can have a step 302 of,prior to steps 308, 310 and 312, heating optical fiber preforms anddrawing the optical fiber preforms into the optical fibers 22 a, 22 band 22 c, using for instance a draw tower. In embodiments where FBGs areto be inscribed along the cores 26 of the optical fibers 22 a, 22 b and22 c, a step 304 of inscribing the FBGs 23 along the cores 26 of theoptical fibers 22 a, 22 b and 22 c can be performed before the steps308, 310 and 312, too. In such embodiments, removing the outer claddingsand/or jackets is not necessary as the freshly drawn optical fibers maybe exempt of such outer claddings and/or jackets. Moreover, it can beanticipated that so-fabricated optical fiber devices may be moresensitive to induced strains, as the absence of outer claddings and/orjackets can allow the optical fibers within the optical fiber devices tobe more flexible.

It will be appreciated that any suitable liquid coating materialdeposition technique can be used. Examples of such liquid coatingmaterial deposition techniques can include, but not limited, extrusion,injection moulding and the like.

FIG. 5 shows an example of a system 500 for fabricating an optical fiberdevice, in accordance with an embodiment. Description of FIG. 5 is madewith respect to the optical fiber device 12 for ease of reading. Asdepicted, the system 500 has a coating material source 504, a die 506,an optical fiber positioner 508, and a coating device 510.

As can be expected, the coating material source 504 has liquid coatingmaterial 40. In some embodiments, the coating material source 504 has aheater melting solid coating material to obtain the liquid coatingmaterial 40.

The die 506 has a longitudinal conduit 512 receiving the optical fibers22 a, 22 b and 22 c extending alongside each other. The optical fibers22 a, 22 b and 22 c are maintained in the geometrical relationship 38relative to one another thanks to the optical fiber positioner 508. Thelength of the longitudinal conduit 512 can vary from one embodiment toanother, as it could be relatively long or short, depending on theembodiment.

In some embodiments, the optical fiber positioner 508 can be partiallyor wholly outside the longitudinal conduit 512 of the die 506. Forinstance, opposite ends 50 and 52 of the optical fibers 22 a, 22 b and22 c can be pulled away from one another, while being maintained in thegiven geometrical relationship 38, by the optical fiber positioner 508,thereby forcing the optical fibers 22 a, 22 b and 22 c to maintain thegiven geometrical relationship 38 all along their lengths duringdeposition of the liquid coating material 40.

However, in this specific embodiment, the optical fiber positioner 508is partially within the longitudinal conduit 512 of the die 506. Morespecifically, the optical fiber positioner 508 includes a nozzle 516having an opening 518 with an inner surface 520 of a given shape, and anouter surface 522 upon which the liquid coating material 40 can flow. Itis intended that the inner surface 520 of the nozzle 516 receives thelongitudinal portions 36 a, 36 b and 36 c of the optical fibers 22 a, 22b and 22 c and confines them into the given geometrical relationship 38during deposition of the liquid coating material 40. As such, thedimension of the opening 518 of the nozzle 516 can be designed to snuglyreceive the longitudinal portions 36 a, 36 b and 36 c of the opticalfibers 22 a, 22 b and 22 c. The snugger the engagement between the innersurface 520 and the longitudinal portions 36 a, 36 b and 36 c of theoptical fibers 22 a, 22 b and 22 c is, the tighter the tolerance can beon the desired geometrical relationship. For instance, in someembodiments, the opening 518 of the nozzle 516 has a dimension below 1mm, preferably below 500 μm and more preferably below 300 μm.

The deposition of the liquid coating material 40 is performed by thecoating device 510 which is in fluid communication with the coatingmaterial source 504 and with the longitudinal conduit 512 of the die506. As shown, the coating device 510 flows the liquid coating material40 towards the longitudinal portions 36 a, 36 b and 36 c of the opticalfibers 22 a, 22 b and 22 c, as they are received in the longitudinalconduit 512 and positioned in the given geometrical relationship 38 bythe optical fiber positioner 508.

As shown, the deposition step can include extruding the liquid coatingmaterial 40 around the longitudinal portions 36 a, 36 b and 36 c of theoptical fibers 22 a, 22 b and 22 c via the nozzle 516 while maintainingthe given geometrical relationship 38. In such an embodiment, theextruding can include a step of forcing the liquid coating material 40at a longitudinal position around the longitudinal portions 36 a, 36 band 36 c and moving the longitudinal portions 36 a, 36 b and 36 c of theoptical fibers 22 a, 22 b and 22 c longitudinally during the forcing ofthe liquid coating material 40. The movement can be in eitherlongitudinal direction. In the embodiment illustrated in FIG. 5, themovement is towards the left-hand side of the page. As shown in thisexample, a pulling mechanism 530 is used to longitudinally pull on atleast the ends 50 of the optical fibers 22 a, 22 b and 22 c. In thisexample, the pulling mechanism 530 also maintains the optical fibers 22a, 22 b and 22 c parallel to one another during the deposition of theliquid coating material 40.

As can be expected, when the longitudinal portions 36 a, 36 b and 36 care moved away from the die 506, the liquid coating material 40 can cooland set up into position, thereby maintaining the given geometricalrelationship 38 between the optical fibers 22 a, 22 b and 22 c. In someembodiments, a cooler is provided to help the liquid coating material 40to set up around the longitudinal portions 36 a, 36 b and 36 c. Thecooler can be a forced airflow cooler, a liquid cooler and/or a Peltiermodule.

FIG. 6 shows the system 500 as part of a larger system 600 performingthe method 300 described above with reference to FIG. 3. As shown inthis example, the system 600 has a draw tower 602 which can heat opticalfiber preforms 60 and draw the optical fiber preforms 60 to form theoptical fibers 22. The system 600 also has an inscription device 604downstream from the draw tower 602 and upstream from the system 500. Asshown in this example, the inscription device 604 is used for inscribingone or more FBGs 23 along a longitudinal portion of each of the opticalfibers 22. In some embodiments, described below, at least one of theFBGs 23 has a random continuous distribution such that a return signalpropagating therealong has a full width at half maximum bandwidthranging between about 0.1 THz and about 40 THz. It is intended that byinscribing the FBGs 23 and/or ROGUE FBGs 23′ along the optical fibers 22directly after the drawing process may circumvent an optional step ofcovering the optical fibers 22 with one or more outer claddings and/orjackets, which can inconveniently impede the flexibility of the opticalfibers 22 and/or the resulting accuracy of the optical fiber device 12in shape sensing applications. A step of removing the outer claddingsand/or jackets can also be omitted. In such embodiments, the body 20 ofcoating material can act as the outer cladding and protect the opticalfibers 22 during use. It is intended that the FBGs can be omitted, asRayleigh scattering may provide sufficient return signal to performinduced-strain measurements using the optical fiber device 12.

Example 1—Extruded Optical Fiber Triplets for 3D Shape Sensing forMinimally Invasive Surgery

Minimally invasive surgery offers a patient the advantage of fasterrecovery and reduces the risks of complications. In order to performsuch surgery, the physician needs to monitor the position and the shapeof the catheter and/or needle being used. Optical fibers have been usedas sensors that allow real time guidance to the surgeon. Optical fibersare widely used for biomedical sensing. With the advantage of being veryflexible and electromagnetically inactive, they can be interesting foruse as sensors in flexible needles and/or catheters and may be used withMRI at the same time. Shape sensors usually rely on fiber strainmeasurement of each fiber of a triplet, as described above. When thetriplet forms a triangle in cross-section, the relative straindifference between the optical fibers at each point provides thecurvature values and directions along the entire length of the opticalfiber device, which can be processed to determine the shape of theoptical fiber device. Strain measurement using the optical fiber devicemay be performed using various methods. The most common method is toincorporate one or more FBGs in the optical fibers and then relate theBragg wavelength variation to the fiber strain. Such a method canachieve high accuracy for simple shapes thanks to the high SNR it mayprovide. In this example, a set of nine FBGs were positioned at threelongitudinal positions along the optical fibers of the fiber triplet,with three FBGs at each longitudinal position. The curvature informationcan therefore be available only at these specific positions (e.g.,typically at 3 strategic points of a 20 cm long optical fiber device),and hence the curvature has to be inferred from these points. Thelimited number of monitored points can induce errors during the shapedetermination, especially when the shape cannot be approximated as asimple function. Another way to obtain the strain of the optical fiberdevice is by using Rayleigh backscatter. The Rayleigh backscatter signalin a fiber can be related to local strain which makes the same processpossible, using OFDR, with the advantage of acquiring distributedinformation along the entire length of the optical fiber sensor. Sincethe Rayleigh signal is usually very low, it can lead to a very poor SNR,and may not allow the addition of in-line optical components for signalprocessing due to poor insertion loss tolerance. It has been shown thatthe backscatter signal can be significantly enhanced by UV exposure,which makes this method promising. In some embodiments, the FBGs areprovided in the form of Random Optical Fiber Gratings written by UV orultrafast laser Exposure, referred to as ROGUE FBGs herein, therebyenhancing backscatter of each of the individual optical fibers. SuchROGUE FBGs have shown a 50 dB enhancement in backscatter in the sensingwavelength range. Knowing precisely the position of each fiber in thesensor can greatly improve bend sensing accuracy.

It is possible to characterize every single optical fiber in thetriplet, and correct the shape measurements accordingly inpost-processing. However, doing so could be inconveniently time- andresource-consuming. There is thus described a method of fabricating thetriplet, which can limit variation in the positioning of the opticalfibers of the triplet along a given length, thereby rendering moot anyprevious characterization steps. In some embodiments, the method caninvolve an extrusion process which can offer precision, as well aspossibility to add various sensors in the body of coating materialsimultaneously, producing a complex composite protected optical fiberdevice in a single extrusion step. Such a fabrication method isparticularly convenient for optical fibers having ROGUE FBGs as theseare generally written into uncoated optical fibers. Indeed, depositingcoating material therearound through an extrusion process can greatlyimprove the durability of the resulting optical fiber device.

This example proposes a process to manufacture optical fiber devices forshape sensors which can be used in a number of biomedical applications.In this example, a polymer extrusion process is performed on threeoptical fibers, thereby forming an optical fiber device with a diameterbelow 600 μm. Accordingly, the optical fiber device can be inserted intosurgical needles, catheters and the like in at least some biomedicalapplications. As described above, the three optical fibers are fixedinto a given geometrical relationship to form a fiber triplet in thisexample. The position of the fiber triplet within the body of coatingand the angle of the fiber triplet are parameters that can beadvantageously controlled to enhance shape measurements. The radial andangular positions of the optical fibers in the triplet are measured withan accuracy of 3 μm and 4 degrees, respectively, in the present example.At least within the context of OFDR measurements, it was demonstratedthat an optical fiber device incorporating optical fibers with ROGUEFBGs could enhance shape sensing.

Similarly to as shown in FIG. 5, the depositing of the coating materialis performed using a twin-screw extruder from Leistritz. As depicted,the die is used in a way that the three optical fibers are coated withthe coating material simultaneously as their relative positioning ismaintained. The lower screw speed that could be reached was 4.2rotations per minute. The fibers were pulled at a speed of 17 cm-s-1.The temperature profile along the die was a humped profile starting at130 degrees Celsius in a first zone, gradually increasing to 195 degreesCelsius in a melt zone and slowly decreasing to 160 degrees Celsius inthe head of the die. The optical fibers used were either uncoated SMF-28(125 μm diameter) incorporating random gratings or polyimide coatedSMF-28 (155 μm diameter) optical fiber. The extrusion tip diameter was660 μm. The head exit had an opening of 940 μm. Once coated, the opticalfibers were cooled using a cooler providing a forced airflow. Thiscooling method can induce a turbulence at the exit of the die which cancompromise the uniform and constant positioning of the optical fibers ofthe triplet. Accordingly, a cooler providing water-cooling was usedinstead to solve the turbulence challenge. The coating material used inthis example is a polymer, and more specifically the M6823MZ from Total,which is a random copolymer made of polypropylene and ethylene. It hasbeen chosen for at least two reasons in this specific example. First,its melting point is at 136 degrees Celsius, which proved to be aconvenient not to damage the optical fibers and/or erase the FBGsinscribed therein. Secondly, its Melt Flow Index (MFI) is 30 g/10 min,which appeared to be an acceptable compromise between a fluidity thatallow the extrusion of a thin enough coating and a melt strength highenough, so the optical fibers do not slip inside the optical fiberpositioner once it has set when the optical fibers undergo strain.

Sensing was performed using three 35 cm long ROGUE FBGs with arespective 45 dB, 50 dB and 25 dB backscatter enhancement on a 5 nmbandwidth centered at a wavelength of 1549.7 nm, as shown in FIG. 7A.The backscatter measurement has been performed using an opticalbackscatter reflectometer (Model OBR 4600 from Luna Innovations Inc).The enhancement backscatter was found to be more than sufficient toenable the three ROGUE FBGs to be connected together to the source ofthe optical interrogated simultaneously using a 1×3 coupler with adifferent delay before each ROGUE FBG. These measurements can be seen inFIG. 7B. The scanning was performed on a 5.24 nm wavelength rangecentered at 1550.5 nm.

Measurement over 20 cm showed that the angle formed by the three opticalfibers was stable within ±2 degrees. Furthermore, the center of thefiber triangle is stable within ±1.5 μm from the center of the body ofcoating material. However, the absolute values may change from oneextrusion to another. FIG. 8A shows a transversal view of an example ofan optical fiber device 800, with an optical fiber triplet 802surrounded by a body 804 of material. Although it can be seen that thetriangle is not necessarily equilateral (which may be an optical formfor shape sensing with three fibers), it is however always isosceles.The triangle shape can be better controlled by customizing the extrusionhead tip. The 600 μm diameter triplet is small enough to go through the950 μm diameter hole catheter. FIG. 8B shows a top view of the opticalfiber device 800. The coating diameter (˜600 μm) has a variability ofaround ±5%. However, even if the triangle formed by the fibers keep thesame shape within the coating and its radial position remains stable, ahuge variability was noticed (more than 30 degrees) in the angularposition of the triangle relatively to the coating center. Theturbulence generated from air cooling at the extrusion head exit couldexplain this observation. Improved results can be expected once asmoother cooling process is set up. An advantage that was noticed wasthe space left in the body of coating material. Such space could befilled with one or more optical fibers, wires and/or capillaries inorder to make a multimodal sensor/surgical tool.

FIGS. 9, 9A and 9B shows another example of an optical fiber device 900fabricated using the method described herein. In this specific example,the optical fiber device 900 has an optical fiber triplet 902 surroundedby a body 904 of coating material. In this example, the optical fibershave a diameter of 155 μm thereby filling more of the opening of theinner surface of the nozzle during the coating material depositionprocess. As the engagement between the inner surface of the nozzle andthe optical fibers is snugger, the optical fibers were more squeezedwithin the inner surface of the nozzle which forced them to be closelyadjoining to one another thereby forming the illustrated equilateraltriangle shape. It is intended that the optical fiber device 900 caninclude at least an additional component 910 relative to the opticalfiber triplet 902 inside said body 904 of coating material. Theadditional component can be one or more of any one of the followinggroup of components: an electrical wire, a conductive glass fiber, acapillary fiber, a photonic crystal fiber, a laser delivery fiber andany other suitable component. FIGS. 10A and 10B show radius and anglevariations along a length of the optical fiber device 900.

Strain measurement using the LUNA OBR4600 shows a stability around of 1%(typically few micro-strain variability for a few hundred micro-strainmeasurement) when repeating a measurement of a same shape. However, thepreviously discussed unknown and random twist of the triplet along thebody of coating material can make the shape reconstruction morechallenging. Regardless, the potential for excellent strain measurementaccuracy will allow highly accurate shape reconstruction results usingthe so-fabricated optical fiber device.

In this example, optical fiber triplets for sensor applications havebeen fabricated using direct polymer extrusion. The extrusion processseems to make the manufacturing of fibers triplet possible at reducedcost since one extrusion can generate an arbitrarily long triplet with agood stability in a single extrusion iteration with respect to geometricparameters. In some embodiments, it is envisaged that a more effectiveand less aggressive cooling method can be used to reduce any angularposition variability of the triplet within the body of coating material.Another advantage of the extrusion process is that it is very flexible.It would be easy to insert more optical fibers, electrical wires sensorsand/or other sensing/surgical tools within the body of coating materialfor additional functionality. A single fiber could be added in order tomeasure the temperature along with a strain measurement in parallel tomeasure ambient changes during shape sensing, which could thereby allowthe strain measurements to be compensated with local temperaturevariations. Another advantage to such an extrusion process is that theavailability of a huge variety of polymers provides a large choice ofcoating properties for specific applications.

For strain measurements, the strong backscattered signal of the ROGUEFBG was found to be convenient. The signal may be strong enough toaccommodate an insertion loss factor of 10 dB (forward and return lossin a 1×3 coupler) while still maintaining a good SNR for accurate strainmeasurement, even when scanning a relatively narrow band (5.24 nm).Furthermore, the simultaneous measurement coupled with the narrow bandscan make the response time such that perform shape sensing in real timemay be envisaged.

Other aspects of the present disclosure may encompass improvementsgenerally relating to distributed temperature and strain optical sensing(hereinafter “DTSS”) systems and more specifically relating tofiber-based DTSS systems.

Typical DTSS systems generally include an optical interrogator which isoptically coupled to a sensing optical fiber. The optical interrogatoris configured for emitting an optical signal along the sensing opticalfiber, and for receiving a return optical signal returning from thesensing optical fiber as the optical signal propagates along the sensingoptical fiber. Measuring temperature change(s) and/or strain change(s)to which the sensing optical fiber are subject can be done by performingseveral measurements over time. In at least some situations, it may beconvenient to monitor temperature change(s) and/or strain change(s)using a plurality of sensing optical fibers to monitor differentenvironments or parts at once. To do so, it is known to use theplurality of optical fibers concurrently with a corresponding pluralityof optical interrogators or to couple the sensing optical fibers to asingle a multi-channel optical interrogator. In some other situations, asingle optical interrogator 1102 is sequentially optically coupled to aplurality of optical sensing optical fibers 1104 using an optical switch1106, as shown in the existing DTSS system 1100 of FIG. 11. Althoughexisting DTSS systems are satisfactory to a certain extent, thereremains room for improvement. For instance, using the two former optionscan be costly whereas the latter option is only limited to sequentialmeasurements.

FIG. 12 shows an example of a DTSS system 1200, in accordance with anembodiment. As depicted, the DTSS system 1200 has an opticalinterrogator 1202, an optical coupler assembly 1204 which is opticallycoupled to the optical interrogator 1202 and a plurality of opticalfiber devices 1206 which are optically connected to the optical couplerassembly 1204.

More specifically, in this example, the optical coupler assembly 1204has an input 1208 being optically coupled to the optical interrogator1202 and a plurality of outputs 1210 to which are optically connectedthe optical fiber devices 1206. As shown, the optical coupler assembly1204 is provided in the form of a two-by-two (e.g., a 50:50) fibercoupler 1212 in this example. This two-by-two fiber coupler 1212 can bereferred to as a one-by-two fiber coupler by at least somemanufacturers. Nevertheless, two-by-two fiber couplers and/or one-by-twocouplers could be used in the DTSS system 1200. Accordingly, the input1208 is referred herein to as an optical fiber input 1208 and theoutputs 1210 are referred to as optical fiber outputs 1210. However,other types of optical coupler assemblies could alternately be used. Forinstance, non-fibered coupler assemblies such as free-space couplerassemblies could be used in some other embodiments.

As such, during use, the optical interrogator 1202 is configured foremitting an optical signal at the optical fiber input 1208 which will bepropagated, at least to a certain extent, along the optical fiberoutputs 1210 and then to the optical fiber devices 1206. The opticalinterrogator 1202 is configured for receiving, in response to theemission of the optical signal, one or more return signals returningfrom corresponding ones of the optical fiber devices 1206. As shown, theoptical interrogator 1202 has an optical emitter 1214 for emitting theoptical signal and an optical receiver 1216 for receiving the returnsignal. Examples of such optical emitter 1214 and receiver 1216 arepresented below. As can be understood, the optical interrogator 1202typically has an internal optical coupler (not shown) so as to allowemission and reception of optical signals at a single optical port 1218,to which the optical fiber input 1208 of the optical coupler 1212 isconnected in this example. In some other embodiments, an opticalcirculator could have been used instead of or in addition to theinternal optical coupler.

In the illustrated example, the optical fiber devices 1206 have firstand second optical fiber devices 1206 a and 1206 b. The first opticalfiber device 1206 a has a first sensing optical fiber 1220 which isserially connected to a first one of the optical fiber outputs 1210 ofthe optical coupler assembly 1204. The second optical fiber device 1206b has an optical path extender 1222 which is serially connected to asecond one of the optical fiber outputs 1210, and a second sensingoptical fiber 1224 which is serially connected to the optical pathextender 1222.

As shown in this example, the optical path extender 1222 is provided inthe form of a length of optical fiber which extends the optical pathlength of an optical signal propagating therein. The optical pathextender 1222 shown in this example can thus be referred to as adelaying optical fiber 1226. However, in some other embodiments, theoptical path extender 1222 can be provided in the form of a multipasscell in which the optical patch is increased by a series of reflectionson a plurality of reflective surfaces or in the form of other types ofoptical path extenders.

More specifically, the delaying optical fiber 1226 has an optical pathlength which is equal to or greater than an optical path length of thefirst optical fiber device 1206 a. In this way, during use, the opticalinterrogator 1202 is configured for receiving, in response to theemission of the optical signal at the optical coupler assembly 1204, afirst return optical signal returning from the first sensing opticalfiber 1220 and a second return optical signal returning from the secondsensing optical fiber 1224.

As can be understood, due to the extended optical path caused by thepresence of the delaying optical fiber 1226 in the second optical fiberdevice 1206 b, the first and second return optical signals aretemporally delayed from one another when they arrive at the opticalinterrogator 1202. Such a configuration can thus allow independentmeasurements to be taken on the first and second sensing optical fibers1220 and 1224 even when using an optical signal having a single opticalpulse, for instance. Examples of such first and second return opticalsignals are shown in FIG. 12A.

Still referring to FIG. 12, the DTSS system 1200 has a controller 1230which is communicatively coupled, via wired communication and/orwireless communication, to the optical interrogator 1202 for processingand/or storing data indicative of the received return optical signal(s).As can be understood, the controller 1230 can be provided as acombination of hardware and software components. The hardware componentscan be implemented in the form of a computing device 1300, an example ofwhich is described with reference to FIG. 13 whereas the softwarecomponents of the controller can be implemented in the form of asoftware application.

Referring to FIG. 13, the computing device 1300 can have a processor1302, a memory 1304, and I/O interface 1306. Instructions 1308 forperforming the method to perform distributed strain and/or temperaturemeasurements can be stored on the memory 1304 and accessible by theprocessor 1302.

The processor 1302 can be, for example, a general-purpose microprocessoror microcontroller, a digital signal processing (DSP) processor, anintegrated circuit, a field programmable gate array (FPGA), areconfigurable processor, a programmable read-only memory (PROM), or anycombination thereof.

The memory 1304 can include a suitable combination of any type ofcomputer-readable memory that is located either internally or externallysuch as, for example, random-access memory (RAM), read-only memory(ROM), compact disc read-only memory (CDROM), electro-optical memory,magneto-optical memory, erasable programmable read-only memory (EPROM),and electrically-erasable programmable read-only memory (EEPROM),Ferroelectric RAM (FRAM) or the like.

Each I/O interface 1306 enables the computing device 1300 tointerconnect with one or more input devices such as the opticalinterrogator 1202, a keyboard, a mouse and the like, or with one or moreoutput devices such as a display, a memory and the like.

Each I/O interface 1306 enables the controller 1230 to communicate withother components, to exchange data with other components, to access andconnect to network resources, to serve applications, and perform othercomputing applications by connecting to a network (or multiple networks)capable of carrying data including the Internet, Ethernet, plain oldtelephone service (POTS) line, public switch telephone network (PSTN),integrated services digital network (ISDN), digital subscriber line(DSL), coaxial cable, fiber optics, satellite, mobile, wireless (e.g.Wi-Fi, WiMAX), SS7 signaling network, fixed line, local area network,wide area network, and others, including any combination of these. Ofcourse, the controller 1230 is optional as it can be omitted in certainembodiments. The computing device 18 of the shape sensing system 10described with reference to FIG. 1 can be similar to the controller 1230described with reference to FIGS. 12 and 13.

Referring back to FIG. 12, it is noted that although the first opticalfiber device 1206 a is shown without a delaying optical fiber 1226, thefirst optical fiber device 1206 a can have a delaying optical fiberconnected between the first one of the outputs 1210 of the opticalcoupler assembly 1204 and the first sensing optical fiber 1220 in someother embodiments. In such embodiments, the optical path length of thedelaying optical fiber of the second optical fiber device 1206 b ischosen so as to be equal or greater than the length of the first opticalfiber device 1206 a, i.e., equal or greater than the length of thedelaying optical fiber of the first optical fiber device 1206 a and thefirst sensing optical fiber 1220 combined to one another.

Although the DTSS system 1200 described with reference to FIG. 12 hasfirst and second optical fiber devices 1206 a and 1206 b, otherembodiments can have more than two optical fiber devices. Such examplesare presented with reference to FIGS. 14 and 15.

FIG. 14 shows an example of a DTSS system 1400 having an opticalinterrogator 1402, an optical coupler 1404 having an optical fiber input1406 connected to the optical interrogator 1402, and first, second andthird optical fiber outputs 1408 to which are connected respective onesof first, second and third optical fiber devices 1410 a, 1410 b and 1410c. The first and second optical fiber devices 1410 a and 1410 b of thisexample are similar to the ones described with reference to FIG. 12. Forclarity purposes, the delaying optical fiber of the second optical fiberdevice 1410 b will be referred to as the first delaying optical fiber1412 a.

As shown, the third optical fiber device 1406 c has a second delayingoptical fiber 1412 b which is serially connected to a third one of theoptical fiber outputs 1410, wherein the second delaying optical fiber1412 b has an optical path length being equal to or greater than anoptical path length of the second optical fiber device 1410 b, includingboth the first delaying optical fiber 1412 a and the second sensingoptical fiber 1414 b. As shown, a third sensing optical fiber 1414 c isserially connected to the second delaying optical fiber 1412 b. As such,during use, the optical interrogator 1402 is configured for receiving athird return signal returning from the third sensing fiber 1414 c in amanner that the first return optical signal returning from the firstoptical fiber device 1410 a, the second return optical signal returningfrom the second optical fiber device 1410 b and the third return signalreturning from the third optical fiber device 1410 c are all temporallydelayed from one another.

Also shown in this example, the optical coupler 1404 is provided in theform of a three-by-three fiber coupler 1416 having one optical fiberinput 1418 and three optical fiber outputs 1420. In other embodiments,four-by-four fiber couplers, five-by-five fiber couplers and/or X-by-Yfiber couplers (where X and Y are positive integers) could alternatelybe used.

FIG. 15 shows another example of a DTSS system 1500, in accordance withan embodiment. In this embodiment, the optical fiber coupler 1504 hasone optical fiber input 1502 and four optical fiber outputs 1504. Inthis way, four different optical fiber devices 1506 a, 1506 b, 1506 cand 1506 d can be connected to respective ones of the optical fiberoutputs 1504.

In this example, the DTSS system 1500 has an optical interrogator 1508,the optical coupler assembly 1504 having the optical fiber input 1502connected to the optical interrogator 1508, and first, second, third andfourth optical fiber outputs 1504 to which are connected respective onesof first, second, third and fourth optical fiber devices 1506 a, 1506 b,1506 c and 1506 d. The first, second and third optical fiber devices1506 a, 1506 b and 1506 c of this example are similar to the onesdescribed with reference to FIG. 14.

Moreover, in this example, the fourth optical fiber device 1506 d has athird delaying optical fiber 1510 c which is serially connected to thefourth optical fiber outputs 1504 of the optical coupler assembly 1504.The third delaying optical fiber 1510 c has an optical path length whichis equal to or greater than an optical path length of the third opticalfiber device 1506 c, including both the second delaying optical fiber1510 b and the third optical sensing fiber 1512 c. A fourth sensingoptical fiber 1512 d is provided in a serial connection to the thirddelaying optical fiber 1510 c. As such, during use, the opticalinterrogator 1508 is configured for receiving a fourth return signalreturning from the fourth sensing fiber 1512 d. Accordingly, the firstreturn optical signal returning from the first optical fiber device 1506a, the second return optical signal returning from the second opticalfiber device 1506 b, the third return signal returning from the thirdoptical fiber device 1508 c and the fourth return signal returning fromthe fourth optical fiber device 1506 d are all temporally delayed fromone another.

In this example, the optical coupler assembly 1504 has a cascade oftwo-by-two fiber couplers 1514 which are connected to one another toprovide the four distinct optical fiber outputs 1504 to which the first,second, third and fourth optical fiber devices 1506 a, 1506 b, 1506 cand 1506 d are connected.

Of course, other exemplary DTSS systems based on the present disclosurecan have more than four optical fiber devices connected to respectiveoutputs of the optical coupler assembly.

Referring back to FIG. 12, the inventors found convenient to provide oneor more scatter increasing devices 1232 along corresponding one(s) ofthe first and second optical fiber devices 1206 a and 1206 b. Indeed, byproviding such devices, the strength of the first and second returnsignals can be increased, which in turn allow increased sensitivity. Forinstance, in the illustrated embodiment, the scatter increasing device1232 of the second sensing optical fiber can help distinguish a portion(low signal) of the second return signal which returns from the delayingoptical fiber 1226 from a portion (high signal) of the second returnsignal which returns from the second sensing optical fiber 1224. Assuch, the DTSS system 1200 can be configured to prevent interferencebetween the low signal returning from the delaying optical fiber 1226and the high signal returning from the second sensing optical fiber1224.

To achieve such a difference in scattering, the material of the scatterincreasing device can be different from the material of the delayingoptical fiber 1226, as these material can have naturally differentscattering properties. In such a case, a sensing optical fiber having amaterial which is different from a material of a delaying optical fiber1226 can act as the scatter increasing device 1232.

An example of how such an increase in scatter can be provided includesdifferent optical fibers with naturally different scattering. This canbe changed, for example, by a choice of material and/or dopants (e.g.,plastic versus silica, different dopants in silica), a choice of dopantconcentration, a choice of quality as low quality optical fibers willhave greater scattering properties and provide a higher signal comparedto high quality optical fibers which will have lower scatteringproperties and provide lower signal, a choice of numerical aperture (NA)as an optical fiber having a higher NA will collect more back-scatteringthan an optical fiber having a lower NA, and/or any other suitable wayto induce a change in scattering properties along between the delayingoptical fiber and the sensing optical fiber. For instance, increasedscatter optical fibers as made by Corning® can have including Titaniaparticles in their core, which enhance light scatter significantly.

An example of how such an increase in scatter can be provided includes,but not limited to, by changing a concentration of defects along thedevice by UV exposure and/or or fs laser pulses, FBG inscription, randomgrating inscription, ROGUE FBG inscription, and the like. Enhancedoptical fibers like the one described in Yan, Aidong, et al.(“Distributed Optical Fiber Sensors with Ultrafast Laser EnhancedRayleigh Backscattering Profiles for Real-Time Monitoring of Solid OxideFuel Cell Operations.” Scientific reports 7.1 (2017): 9360) could alsobe used as scatter increasing devices.

FIG. 16 shows an example of a scatter increasing device 1600 similar toone of the scatter increasing devices 1232 of FIG. 12. Morespecifically, the illustrated scatter increasing device 1600 is anoptical grating 1602 inscribed along a portion of a corresponding core1604 of a sensing optical fiber 1606. Reference numeral 1608 shows acladding of the sensing optical fiber 1606. The inscribed opticalgrating 1602 has a random continuous distribution such that a returnsignal, caused by propagation of an optical signal therealong, has afull width at half maximum (FWHM) bandwidth ranging between about 0.1THz and about 40 THz. In some embodiments, the FWHM bandwidth canpreferably range between about 0.35 THz and about 7 THz.

In some embodiments, the FWHM bandwidth can be related to a coherencelength of the grating, and can be defined as the length where thevisibility of fringes would drop to 1/e of its initial intensity if thecoherent wave in the grating was to interfere with itself at anotherlocation. The coherence length is proportional to the inverse of theFWHM bandwidth of the return signal, depending on the spectral shape ofthe scatter. More specifically, the coherence length can be equivalentto the following equation:

$\begin{matrix}{{L_{c} = \frac{c \cdot \lambda^{2}}{n \cdot {\Delta\lambda}}},} & (1)\end{matrix}$

wherein L_(c) denotes the coherence length, λ denotes the wavelength ofthe optical signal propagating in the grating, C is dependent on theshape of the spectrum of the return signal, n denotes the refractiveindex of the optical fiber and Δλ denotes the FWHM bandwidth.

Accordingly, in some embodiments, the grating can have a coherencelength which ranges between about 2λ and about 500λ when the returnsignal has a scattering spectrum with a Gaussian shape. In some specificembodiments, the coherence length ranges between about 10λ and about100λ. This can correspond to a back-scattering FWHM bandwidth of about 1nm to 500 nm (at a wavelength of 1550 nm) depending on the scatteringspectral shape. In other words, the back-scattering structure is lesscoherent then a uniform FBG, but more coherent than a Rayleigh scatterstructure. It was found that such gratings can cause return signals tohave a FWHM bandwidth which is constant, provided that the grating has alength which exceeds a given length. In some embodiments, the givenlength is in the centimeter range. As shown, the scatter increasingdevice can be inscribed in the core of the sensing optical fiber butalso in the cladding(s) of the sensing optical fiber.

In some embodiments, the random continuous distribution of the gratingis a random phase distribution. In some embodiments, the randomcontinuous distribution of the grating is a random period or wavelengthdistribution. In alternate embodiments, the random continuousdistribution is a random amplitude distribution. In still furtherembodiments, the random continuous distribution can involve a randomphase distribution, a random period distribution and/or a randomamplitude distribution.

As will be described below, the optical grating, which is also referredto as a ROGUE FBG in this disclosure, has a length which is in thecentimeter range. For instance, the length of the grating can be greaterthan 1 cm, and preferably greater than 2 cm. In some embodiments, thegrating can be inscribed using an inscribing wavelength which isdifferent from a wavelength of the optical signal which is meant to bepropagated therein. However, in some other embodiments, the grating canbe inscribed using an inscribing wavelength which corresponds to thewavelength of the optical signal which is meant to be propagatedtherein. For instance, using a point-by-point non-interferometricinscription technique with a fs pulsed laser, the inscription of thegrating is independent of the inscribing wavelength, but rather onlydepends on a repetition rate of the pulsed fs laser and on its speed.

Example 2—Scatter Based Order of Magnitude Increase in DistributedTemperature and Strain Sensing by Simple UV Exposure of Optical Fiber

An example is presented to improve signal strength, and thereforeincrease sensitivity in DTSS by Fourier domain scatter. A simple UVexposure of a hydrogen loaded standard SMF-28 fiber core is shown toenhance the back-scattered light dramatically by ten-fold, independentof the presence of a Bragg grating, and is therefore created by the UVexposure alone. This increase in back-scatter allows anorder-of-magnitude increase in sensitivity for DTSS compared toun-exposed SMF-28 fiber used as a sensing element. This enhancement insensitivity is effective for cm range or more sensor gauge length, belowwhich is the theoretical cross-correlation limit. The detection of a 20mK temperature rise with a spatial resolution of 2 cm is demonstrated.This gain in sensitivity for SMF-28 is compared with a high Ge dopedphotosensitive fiber with a characteristically high NA. For this latter,although of less amplitude, the UV enhancement is also present, andenables a yet even lower noise level of sensing, due to the fiber'sintrinsically higher scatter signal.

DTSS systems are extremely useful for industrial monitoring, since theyprovide real-time information along a region of interest with low-costoptical fiber. Optical time domain reflectometry (OTDR) using scatterhas been used for decades to investigate distributed information along afiber. It has been demonstrated for DTSS in long lengths of fiber (˜km),but with poor spatial resolution (˜m) and very poor temperaturesensitivity (˜10° C.). On the other hand, it's Fourier Domain (OFDR)counterpart gives the highest spatial resolution in DTSS (˜mm) whileallowing a reasonable temperature sensitivity (0.1 to 1° C.) andremaining a rather simple and cheap scheme, compared to other DTSSschemes. However, Rayleigh scatter OFDR has remained quite limited interms of sensing length (30-100 m). For this reason, other methods havebeen developed using Raman scattering (ROTDR), with allows much longerreach of 1-30 km, and Brillouin scattering (BOTDR or BOTDA), with evenlonger lengths of 10-100 km. Both of these techniques however show lesssensitivity (˜1° C.) and much poorer resolution (1-10 m). Combinationsof technique have also been proposed: Rayleigh and Brillouin scattering,also known as Landau-Placzek ratio analysis, and Rayleigh with Ramanscattering.

The main limitation in the sensitivity and accuracy of Rayleighscattering DTSS comes from the low scattering signal at the detector.Higher scattering medium, such as liquids in hollow core fibers, polymerfibers with larger scattering cross-sections, or specially designed highscattering silica fibers doped with various impurities can be used toincrease this signal, thus increase the sensitivity. However, suchschemes are non-standard and therefore expensive to produce and torender compatible with available optical equipment. There is suggested asimple and affordable method to radically improve temperature and strainsensitivity by ten-fold through a dramatic increase in scatter instandard fiber. This increase comes from simply exposing the fiber coreto UV light, which creates a high density of scattering defects, such asobserved by Johlen et al. in their study of UV exposure induced losses.Such enhancement in fiber can be easily induced with any UV laser (solidstate, argon) without any critical alignment or vibration control unlikewhen writing FBGs. The UV-exposure is also compared to UV writing ofFBGs.

ODFR allows the measurement of a reflectivity pattern, such as Rayleighscattering along a fiber length. The back-scattering effects of Rayleighare caused by defects causing a local variation in the permittivity. Asdescribed by Froggatt et al., such a permittivity can be measured withthe knowledge of the spectral intensity of an interference between thefiber under test and a reference arm:

$\begin{matrix}{{\Delta{\overset{\_}{ɛ}(x)}} = {i\frac{n}{E_{0}^{2}{rc}\;\beta_{0}\pi}{\int\limits_{- {\Delta\beta}}^{\Delta\beta}{{I_{d}\left( {\beta - \beta_{0}} \right)}e^{{- i}\;{{\beta 2}{({x - x_{0}})}}}d\;\beta}}}} & (2)\end{matrix}$

Where I_(d) is the measured spectral intensity of the interference, n isthe refractive index, E₀ the input laser field, r the reflectioncoefficient of the reference beam and x₀ the position of the referencereflection. Considering that the system can have discrete sampling, thecorresponding integral in Eq. (2), the reflected intensity vs position(therefore in the time domain), can be re-written as:

$\begin{matrix}{\overset{\sim}{I} = {\frac{1}{N}{\sum\limits_{k = 0}^{N - 1}{I_{k}e^{{- {ikm}}\frac{2\;\pi}{N}}}}}} & (3)\end{matrix}$

Where N is the total number of points within the measurement and I_(k)is the spectral intensity at different point k along the scan. Themeasurement of temperature/strain is relative to a referencemeasurement. Both are compared by doing a cross-correlation over anintegration length Δx, which is called the sensor gauge length andcorresponds to the spatial resolution of the DTSS:

$\begin{matrix}{{I_{k}^{({ref})} \otimes I_{N^{\prime} - k}^{{({test})}*}} = {\frac{1}{2\pi\; N}{\overset{m_{2}}{\sum\limits_{m = m_{1}}}{{\overset{\sim}{I}}_{m}^{({ref})}{\overset{\sim}{I}}_{m}^{({test})}e^{{ikm}\frac{2\pi}{N^{\prime}}}}}}} & (4)\end{matrix}$

Where N′ corresponds to the number of points in the integration lengthΔx (as N′=m₂−m₁+1), which is considered as the sensor length, or gaugelength, and corresponds to the spatial resolution of the DTSSmeasurement. This cross-correlation is in the Fourier domain. When thereis no strain or temperature change, a peak is expected at zerofrequency. When a temperature or strain change is applied in the sensorgauge length, then this peak shifts proportional to the change.Therefore, the resulting frequency shift is a direct measurement of theobserved change in temperature or strain and such a value can becalculated for every sensor gauge lengths Δx. It is desirable tominimise this length, since it defines the spatial resolution. However,the longer the sensor length, the higher the peak intensity in thecross-correlation, giving a higher signal to noise ratio (SNR). Thenoise itself is intrinsic to the calculation and the nature of Rayleighscattering, i.e. the random fluctuation in Δε(x), therefore is alwayspresent. This being said, the longer the gauge length, the moreprecisely can the frequency shift be determined, thus improving theprecision of the temperature or strain measurement. The link betweenfrequency shift and strain or temperature is determined by a calibrationconstant within the OFDR system. It can be shown that such a physicaluncertainty limit can be express as follows:

$\begin{matrix}{{{\Delta\; x\; ɛ_{res}} = \frac{\lambda}{4\; n}}{{\Delta\;{xT}_{res}} = {\frac{\lambda}{4\; n}\left( \frac{d\; ɛ}{dT} \right)^{- 1}}}} & (5)\end{matrix}$

Where ε_(res) is the strain resolution (dimension-less) and T_(res) isthe temperature resolution. This theoretical limit can be easilyobserved in the results of FIGS. 17A-C for small gauge lengths. However,at longer lengths of Δx (1 cm or more), another limit appears: thedetector intensity noise (not taken into account in Eq. (5)) and thetemperature fluctuation along the gauge length.

To improve these limits, there is presented that a simple UV exposure onthe fiber, such that a very weak out of band reflection grating iswritten has the effect of increasing the scattering emission andcollection by ten-fold, thus generating an increase in sensitivity. UVexposure of hydrogen (or deuterium) loaded SMF-28, as well as on highgermanium content core fiber has the effect of creating colour centers.However, why such exposure would increase scatter, which normally comesfrom defects in a fiber, is still under investigation. However, thepresence of the weak grating away from the wavelength of measurementimproves the scatter signal dramatically. An increase in back-scatteringcollection can also be accounted for by an increase in the NA of thefiber. However, this increase in collected back-scattered power isexpected to be limited to a factor of <2, for the expected refractiveindex change, below 10⁻³ in our demonstration.

Two types of fibers were tested: a standard SMF-28 single-mode telecomfiber by Corning which was hydrogen loaded for increasedphotosensitivity and a high NA (0.27) photosensitive fiber with high Gecontent from CoreActive (uvs-eps), referred to as HNA in this example.in the latter fiber, exposure was made by continuously exposing thefiber with UV light, while with the SMF-28, two types of FBGs weretested and compared with the continuously UV exposed fiber: a uniformgrating and a random grating with randomly positioned phase shifts. FBGswere measured out-of-band for DTSS, to not be limited by the grating'sbandwidth. Although the FBGs is expected to offer more back-scatteringthan simple UV exposure since there is more refractive index variationwithin the fiber, surprisingly, results show that this is not the case,as can be seen in 7B. Indeed, a continuous UV exposure generates a 20 dBintensity increase in back-scattering return signal, which correspondsto a ten-fold increase in local losses, when taking into account theround-trip nature of the measurement which squares those local losses.Back scattering in the presence of the grating, uniform or random, alsoincreases by the same order of magnitude. Although a more complexstructure (oscillations) is observed when a random grating is involved,it was noted that this did not lead to any gain in strain or temperaturesensitivity. Indeed, whether a grating is present or not, the averageexposure is the same, and an identical gain was observed in terms ofdistributed temperature or strain noise. However, it is suspected thatthere may be a small contribution from the presence of the FBG, althoughthis would require further investigation.

The UV-exposed HNA fiber shows the same improvement in scatter signal,compared to SMF-28, as can be seen in FIG. 17A. However, when comparingthe effect of the exposure itself, i.e. the difference in signal betweennon-exposed HNA and exposed HNA fiber, the gain is not as great sincethis fiber already has a rather high Rayleigh signature (˜three timesthat of SMF-28) before exposition. This signature can be due to itsfabrication inducing more defects as well as from its higher NA, whichallows more collection of the back-scatter. Nevertheless, as seen inFIG. 18B, this UV-exposed fiber exhibits a DTSS noise level even moreadvantageous than UV-exposed SMF-28.

The gain in back-scattered signal gives rise to a considerable increasein temperature and strain sensitivity. Indeed, with this improvement insignal to noise ratio, the cross-correlation of Eq. (4) yields a moreprecise frequency shift, thus a higher sensitivity in temperature orstrain measurement. Note that these measurements are in temperature, butthe same picture can be applied to strain with a factor of 8.32 με/° C.(calibration factor for silica fiber from the LUNA system). Thecollected back-scatter was measured with varying UV exposure power, asshown in FIG. 18, to understand the optimal exposure to minimiserequirements and maximise gain in sensitivity. As can be seen, after arather linear increase, the gain saturates at around a ten-foldincrease. Our choice of power and exposure time for DTSS tests werebased on the best exposure conditions. The mechanics of such increaseand saturation are still under investigation.

A quantitative analysis is shown in FIG. 19 where the RMS noise levelwas calculated based on a 30 cm section of 1 mm spaced points. Thesensor cross-correlation integration length, which defines the spatialresolution of the DTSS, was varied from a long length of 10 cm to a veryshort length of 1 mm. The limit defined by Eq. (5) can be observed inthese results for short sensor lengths. The higher SNR of the UV exposedfiber seems to slightly improve the resolution within this range, wherenoise is limited by the cross-correlation on a random structure, typicalof scatter. The most important gain is in the cm range, where thedetection noise, unrelated to backscattering from the material structureand defects, becomes the dominating limitation. In this same range, thenoise level can be expected to rise slightly as the sensor lengthincreases, since it becomes more sensitive to thermal fluctuation alongits length. However, the sensing fiber tested here was in a thermallystable isolated container, which explains why the thermal noise is morestable and actually decreases slightly with sensor length. This showsthe performance limit of the DTSS system itself, independent of theenvironment. From these results in 9, there is showed a RMS noise levelof 10-15 mK in temperature for gauge lengths of 2 cm or more forUV-exposed SMF-28. If converted to strain, this gives a RMS noise levelof 80-120 nε. The UV-exposed HNA fiber's performance is twice as goodcompared to the SMF-28 with an RMS noise level of ˜5 mK (40 nε), thebest performance to date with this resolution level.

The difference in noise and ease of measurement can be appreciated inFIG. 10A, where we can see the improvement in the measurement along thefiber, which is placed in an isothermal, stable and isolated container.Another measurement example is shown in FIG. 9B where the resolution andsensitivity can be appreciated. In this case, a thin 0.2 mm diameterwire heated by a low current of 20 to 100 mA is placed in contact with afiber (UV-exposed SMF-28) in a perpendicular fashion. With a 2 cm sensorintegration length, the point-spread-function of such a DTSS measurementmay be observed. The heating by 20 mK may be seen in the bottom curve inFIGS. 20A and 20B. To further increase the resolution and measurementquality, a spatial averaging was performed on surrounding points. Theequivalent length of this spatial averaging was chosen as half thesensor integration gauge length so as to not further limit the spatialresolution.

While it has been nominally noted in the past that UV exposure increasesback scatter in optical fiber, there has been no systematic study toeither understand it or to apply it. In this example, we have undertakento find the conditions to maximise scatter through UV radiation and alsowith the writing of weak off-band Bragg gratings, and then to use theincrease in what we believe is the first application in sensing. We haveshown here that a simple continuous UV exposure of a hydrogen loadedSMF-28, increases the back-scattered intensity by ten-fold, thusallowing a ten-fold increase in DTSS sensitivity. Increase in thecollected back-scatter from a gain in the NA of the fiber can accountfor a factor of only 2 for the exposure used here, therefore theremaining back-scatter signal gain comes from an increase in scatteritself. The reason for this increase is still under investigation.However, with the presence of a weak off-band Bragg grating in theoptical fiber, the scatter increases dramatically due to the nature ofside modes of the grating. The increase in the signal, greatly improvesthe SNR at the detector, therefore pushing down the noise floor in DTSSmeasurements to the theoretical sensitivity/spatial resolution limit.Indeed, for a sensor integration length of 2 to 10 cm, a RMS noise levelof 10 mK or 80 nε was obtained in a thermally stable environment instandard UV exposed H2 loaded fiber, after the removal of the hydrogen.An even lower noise floor was shown with a high NA photosensitive fiberto 5 mK or 40 nε, the best reported performance, to our knowledge, for a1-2 cm range gauge length. In comparison, Gifford et al. demonstratedrecently a 1 mK resolution, but with a 12 cm gauge length using weaksemi-continuous FBG to increase return signal. However, when using Bragggratings in-band, one is limited by the band-width of the grating, thuslimiting the spatial resolution. Since UV-exposure affects the entirespectrum of scatter, our method of improvement does not involve anytheoretical bandwidth limit, except equipment limitation from the scanrange and practical consideration of measurement time.

With a saturating exposure in standard fiber, we can expect to furtherpush back this limit to 5 mK noise level for a 2 cm sensor and perhaps 1mK for a 10 cm spatial resolution, although at this stage, applicationsare limited to a very stable environment. Improving back-scatter is verysimple, since it only requires a UV laser and a focusing element. Nocritical alignment or vibration stabilization is required. Although weused hydrogen loaded SMF-28 in our demonstration, the same effect wasshown in photosensitive fiber exhibiting a similar UV interactionmechanism, i.e. color center generation, such as highly doped germaniumfiber. Therefore, UV exposure can be performed easily during the drawingprocess in such a photosensitive fiber before the coating phase. It isalso a much easier technique to improve sensitivity than writingmultiple gratings along the sensing fiber, which does increasesensitivity, while sacrificing spatial resolution and increasedfabrication costs.

Continuous UV exposure was performed using our high precision FBGwriting system without writing and with writing a weak off-band FBG. Thefiber core was illuminated with 213 nm wavelength from the 5^(th)harmonic of a 1064 nm solid-state laser (from Xiton Photonics GmbH). ForDTSS tests, the fiber was exposed with 50 mW of power at 50 μm persecond with a spot size of ˜200 μm giving a uniform exposure time of ˜4seconds. Continuous UV exposure was compared to an out of band OFDRsignature of FBGs written with the same exposure time and power, by adirect holographic writing technique using the same experimental setup.For scattering characterisation tests with exposure, power or speed wasvaried along a length of 100 mm. DTSS was performed using a commercialOFDR system from LUNA. Cross-correlation to resolve the frequency shiftwas also performed by the same commercial system. The sensing fiber wasplaced in a thermally stable environment (insulated box) to eliminatethermal fluctuations to provide a real sense of the measurement's noiselimits. The long gratings were interrogated out-of-band to allow maximumpenetration of the input light and to ensure a maximum sweeping range ofthe OFDR system. In such a case, it is not the grating resonance that isused, but the microscopic index variation due to the periodic nature ofthe refractive index modulation and which generate enhancedback-scatter. In order to ensure there is no contribution to the scattermeasurements from the molecular hydrogen in the fiber, all measurementswere in the following weeks after the UV exposure to allow the hydrogento diffuse out at room temperature.

Example 3—Method of Inscribing an Optical Grating

The ROGUE FBG was written in the fiber using a Talbot interferometerbased FBG writing station. The fiber core is exposed to a 213 nmwavelength laser, using the 5^(th) harmonic of a 1064 nm laser. FIG. 21presents this setup. The first two orders (+1 and −1) are reflected bymirrors onto the fiber, where an interference pattern is created. Thisinterference pattern consists of very small regions on the fiber wherethe UV power is high, followed by regions where the UV power is low, ina periodic fashion. The UV exposure increases the refractive indexlocally, creating an FBG. By changing the angle of the mirrors, theinterference pattern step (and thus the FBG wavelength) can be modified.

When writing a long FBG, the fiber is moved continuously under the phasemask. In order to preserve the interference pattern on the fiber, asawtooth electric wave is applied to a piezoelectric element moving thephase mask at the same rate as the fiber, and then bringing it backafter a certain movement amplitude. If the sawtooth frequency andamplitude match the FBG period and fiber moving speed, the interferencefringes will overlap and a continuous, very high quality FBG will bewritten in the fiber. If the sawtooth wave does not have the rightfrequency or amplitude, the interference pattern will erase itself,since the entire length of the fiber will be exposed to the UV light,instead of specific interference fringes. However, the UV exposure isnot completely uniform because of noise in the system, and a randominterference pattern will appear in the fiber, leading to reflectivityover many different wavelengths. This reflectivity pattern is similar tohaving a very weak, very broadband Bragg grating all along the entirelength of the fiber. However, it can be easily modeled as a multitude ofvery small, randomly placed gratings, so that the ROGUE FBGs bandwidthwill stay the same as its length increases, contrary to a uniform Bragggrating, whose bandwidth decreases as the length is increased.

In order to increase the reflectivity of the ROGUE FBG, all that isneeded is to increase the noise and slow down the rate of movement ofthe fiber. In order to do so, we simply replace the sawtooth wave by arandom electric signal. This way, we are not dependent on the noise inthe system but can generate the noise ourselves, increasing thebackscattered signal by orders of magnitude.

Another way to make such a ROGUE FBG would be to place the phase maskclose to the surface of the fiber, instead of having a Talbotinterferometer configuration, as shown in FIG. 22. The diffractionorders +1 and −1 directly interfere and form a near-field fringe patternon the optical fiber, generating an FBG. The disadvantage of thistechnique is that the FBG central wavelength cannot be changed, unlessthe phase mask is moved as in the case of the Talbot scheme with apiezoelectric element. The wavelength can be changed by the speed of thefiber relative to the movement of the piezo. However, a ROGUE FBG can bewritten in a similar way as with the Talbot interferometer, by movingthe fiber and applying a random electric signal on the piezoelectric.

Two different optical fibers were studied: standard SMF-28 optical fiberfrom Corning, the most widely used fiber in telecommunications, and aSM1500 highly Germanium doped fiber from FiberCore, an intrinsicallyphotosensitive fiber with more than 5 times more Germanium than standardoptical fibers. Both fibers were loaded with molecular hydrogen in orderto increase photosensitivity.

Using the setup explained in the previous section, we wrote ROGUE FBGswith a backscatter intensity ranging from under 5 to over 50 dB abovestandard SMF-28. FIG. 23 show the results of such a ROGUE FBG in both,the temporal and spectral domains, written in SMF-28 fiber. A 45 dBincrease in backscattered amplitude can be observed over the signallevel of the unexposed fiber in the ROGUE FBG in the temporal domain inFIG. 23A. The reflection spectrum of the ROGUE FBG is shown in FIG. 23B.Its impressive spectral width (48 nm full width, 7 nm full width at halfmaximum) generates an important increase in signal over a very widerange of wavelengths.

By modifying the random signal amplitude and frequency applied to thepiezoelectric element, the ROGUE FBG writing speed and the laser power,the strength of the ROGUE FBG can be modified and optimized. From ourexperiments, an amplitude of 5 V (the maximum that could be applied toour piezoelectric unit, corresponding to about 10 periods) and afrequency of 20 Hz yielded the best results. For higher frequencies, theROGUE FBG amplitude decreases, most likely because the piezoelectricelement cannot follow the random signal that is applied to it, thusreducing the effective amplitude of its movement. The writing speed andpower was found to influence the strength of the ROGUE FBGsignificantly, as shown in FIG. 24. In this figure, 0 dB corresponds tothe signal level of unexposed SMF-28 fiber. Measurements were made onthe OBR 4600 using a 21.16 nm scanning bandwidth. The scanning bandwidthwe established was best for sensing (as will be shown in the nextsections).

The noise levels of both, standard, unexposed SMF-28 fiber and the ROGUEFBGs were measured by placing the fiber inside an insulated box, inorder to avoid environmentally induced perturbations, such as aircurrents. In order to avoid problems related to the OBR's dynamic range,two different fibers were used, one in which a ROGUE FBG was inscribed,and another one that was left untreated. Indeed, during ourexperimentations, we realized that the inscription of a ROGUE FBG overone part of the fiber influenced the measurements on the rest of thefiber. For both fibers, a 30 cm sensing range was selected, and thespectral shift was calculated at every 1 mm along this sensing rangewith a 1 cm gauge length, leading to 300 sensor points per fiber. FIG.25 presents the results of the root mean square (RMS) error of bothfibers, calculated over all sensor points in the selected part of thefiber, and averaged over 15 measurements. In FIG. 25, results directlytaken from the OBR are compared to the results after adding a correctionto the OBR data treatment (the nature of the correction is beyond thescope of this report). All measurements relate to the sensing of strainor temperature variations and are measured in GHz. For SMF-28, 1 GHzcorresponds to a variation of temperature of 0.801° C. or to a strainvariation of 6.668με (a stretching or compression of 6.668×10⁻⁴%).

From FIG. 25, it can be observed that, for the unexposed fiber, the RMSerror decreases when the bandwidth is increased. That is to be expected,since an increase in bandwidth leads to a higher signal to noise ratio(SNR) However, surprisingly, the opposite happens in the case of theROGUE FBG. That is most likely due to the fact that, contrary to theunexposed fiber, most of the signal comes from a very narrow spectrum.As such, the SNR actually increases when the scanning bandwidth issmaller, because only the portion of the spectrum of the ROGUE FBG isprobed during the laser scan.

In order to characterize the accuracy of our sensors, we slowlystretched a 1.15 m fiber in increments of 1 μm using our nanometerprecision stage (air bearing stage from Aerotech), and compared thestrain measured by the sensor to the calculated value. All the scanningbandwidths offered by the OBR were tested, and both, the ROGUE FBG andthe unexposed SMF-28 fiber were compared with the calculated values.Again, two different fibers (one in which a grating was inscribed, theother without) were tested separately, so that the grating did notinfluence the measurement on the unexposed fiber. The results are shownin FIGS. 26A-F. As can be seen, the ROGUE FBG yields far more accurateresults than the unexposed SMF-28. Furthermore, the ROGUE FBG remainsvery similar to the calculated values for very short scanningbandwidths, and it is not until the smallest scanning bandwidth of 1 nmis used that significant errors can be observed. Measurements were takenwhile the fiber was static, in between stage movements, so that thestrain remains constant during the frequency scan.

From these measurements, the root mean square errors between calculatedand experimental values of the spectral shift were calculated over the20 μm stretching. In the algorithm, an 8 cm sensing region wasestablished on both, the ROGUE FBG and the unexposed SMF-28 fiber, withsensors at every 1 mm (80 sensors total). The RMS error was calculatedacross all those sensors, over the 20 μm stretching. As FIG. 27 shows,the behavior for the ROGUE FBG is very similar to the unexposed fiber,but the error is significantly lower, almost by an order of magnitudefor every scanning bandwidth. In the case of the SMF-28, the error isminimal for the largest bandwidth, which is to be expected, but in thecase of the ROGUE FBG, we see that the minimal error is actually for a21.16 nm bandwidth. The explanation for this is the same as for thenoise level, which is that since most of the signal is from a narrowerbandwidth, a larger bandwidth does not necessarily yield a more accuratemeasurement. At this bandwidth, the RMS error is 4.5 smaller than withstandard SMF-28. It can be observed that, using a ROGUE FBG, a scanningbandwidth of only 5.24 nm is sufficient to beat the best accuracyobtained with the SMF-28.

Finally, we evaluated how the strength of the ROGUE FBG influenced theaccuracy of the measurement. In order to do this, ROGUE FBGs of variousstrengths were written, and the same measurements were performed over 20μm stretching. Only the 21.16 nm bandwidth (the one that showed thesmallest error) was used for the scan. FIG. 28 shows how the error isinfluenced by the ROGUE FBG gain. Unsurprisingly, for stronger gratingsthe mean error decreases. The small increase in the error as the gain ofthe ROGUE FBG increases is currently under investigation. However, it isnoted that by decreasing the writing speed of the ROGUE FBG, it becomesstronger, to the cost of a slightly smaller bandwidth.

In order to characterize the ROGUE FBGs ability to compensate for lossin the system, we repeated the same experiment with the three strongestROGUE FBGs while inducing optical loss before the grating, to see howthat affected the spectral shift accuracy. FIG. 29 presents theseresults as a function of loss in measurement signal, in which theoptical loss was induced by a variable optical attenuator placed beforethe ROGUE FBG. As increasing loss is induced before the grating, theerror increases slowly until a threshold is reached and a catastrophicincrease in error occurs. This occurs when the loss gets close to theROGUE FBG enhancement value. As such, the SNR is decreased below therequirement for proper cross-correlation. When the noise level has thesame amplitude as the maximum of the ROGUE FBG reflectivity, all theinformation is lost in noise, and the algorithm can no longer recoverthe spectral shift.

Using the ROGUE FBG fabrication technique described earlier, we wereable to increase the backscattered signal by orders of magnitude, i.e.to over 50 dB above standard SMF-28 levels. This increase in signalturns into an improvement of over an order of magnitude in RMS noiselevel, and an RMS error on accuracy in strain measurements 4.5 timessmaller than standard SMF-28 fiber. As such, the most accurate andprecise ROGUE FBGs we were able to fabricate, exhibited an RMS noiselevel of 0.016 GHz (0.1 pc or 13 mK) and 0.05 GHz spectral shift RMSerror (0.34 pc or 40 mK) for a stretching range from 0 to 20 μm of the1.15 m fiber length (0 to 17.4 pc). It is important to note that, forthese last measurements, the fibers were placed inside a closed spacethat did not provide an environment as controlled as the insulated boxthat was used in the other set of experiments. As such, temperaturefluctuations of this magnitude can be expected, and even more accuratereadings could thus be expected when placing the fibers in a bettercontrolled environment. Even though we showed that increasing thebackscatter by more than 25 dB does not seem to improve measurementaccuracy, for cases where significant loss occurs in the system, it isstill worthwhile to increase the backscatter above 50 dB, in order tocompensate for that loss. In case of loss, there is little wiggle roomavailable with untreated standard SMF-28.

Since the scatter enhancement technique relies on noise during thewriting process, the optical alignment is not critical, and theexperimental conditions and the equipment do not require extensivecontrol. Relatively fast writing speeds of 1 or even 10 mm/s yieldsufficient increase in backscatter to noticeably enhance the accuracy ofsuch sensors without requiring an enormous amount of laser power. Theexperimental setup used in this example relies on a Talbotinterferometer configuration but could as easily be used by directinscription of the grating with the fiber directly behind the phasemask. As such, this technique could thus be implemented in an industrialassembly line, or even during the drawing process of the fiber. Arguablyenvironmental fluctuations or equipment vibrations during the ROGUE FBGwriting could potentially even increase the strength of the ROGUE FBG byadding other sources of noise.

It is being noted that this enhancement is limited to the bandwidth ofthe ROGUE FBG. The laser power and writing speed are not sufficient togenerate uniform Rayleigh enhancement, and as such scanning outside theROGUE FBG does very little to enhance backscatter. However, scanningbandwidths of 21 nm and under are typically more than enough for mostapplications, and smaller bandwidths are usually favored for real-timeapplications, because they allow faster acquisition speeds.

Example 4—Order of Magnitude Increase in Resolution of Optical FrequencyDomain Reflectometry Based Temperature and Strain Sensing by theInscription of a ROGUE FBG

OFDR has been investigated for two decades as a way to replace the FBGcurrently used in most industries for sensing applications, using theintrinsic Rayleigh scatter of fibers instead. OFDR allows completelydistributed strain and temperature measurements along a fiber. Theincrease of backscatter using UV laser exposition was recently reported,and was found to increase the sensitivity in both temperature and strainsensing. We present a technique that increases the backscattered signalamplitude by over 50 dB, based on the writing of a ROGUE FBG, i.e. avery weak, random grating over the entire length of the fiber. Thisimprovement is, to the inventors' knowledge, over 25 dB higher than whatwas previously reported for UV exposure for the same exposition power.The ROGUE FBG is generated by inducing phase noise during the continuouswriting of a FBG using a Talbot interferometer. This leads to a gratingwith a very broad bandwidth regardless of the exposure length andgreatly increases the signal without limiting the scanning bandwidth,resulting in no loss in resolution. Using these enhanced fibers, weobtained a noise level over an order of magnitude lower than usingregular unexposed fibers, allowing measurements of smaller temperaturevariations. Fibers where such ROGUE FBGs are inscribed also allow theuse of a much smaller scanning bandwidth with similar accuracy,resulting in faster acquisition speed.

In this example, we present a technique increasing the backscatteredsignal for Rayleigh scatter based distributed sensing by several ordersof magnitude. This is achieved by writing a ROGUE FBG (a weak, randomFBG) over the entire length of the fiber, resulting in higherreflectivity over a wide range of wavelengths. This in turn leads tobetter resolution in strain and temperature measurements when comparedwith standard single-mode fiber with uniform UV exposure, and to thepossibility of faster scan speeds by using a narrower scanning bandwidthwith similar accuracy.

Example 5—Influence of the Length of a ROGUE FBG on its FWHM Bandwidthand on its RMS Noise Level

In this example, the reflectivity and bandwidth of multiple lengths ofROGUE FBGs are measured. The results are presented in FIGS. 30A and 30B.As can be seen, the bandwidth of the grating is initially very large,but quickly converges to a certain value (here, about 7 nm) as thegrating's length is increased. A probabilistic model was developed toevaluate the ROGUE FBGs behaviour for longer gratings. As can be seen,the ROGUE FBG bandwidth remains constant as the grating's lengthincreases, while the reflectivity keeps increasing. As can beappreciated, a satisfactory agreement between the modeled andexperimental data can be observed in FIG. 30B.

The influence of the gauge length on the noise level is also evaluatedand presented in FIG. 31. It was found that a smaller gauge length leadsto a higher spatial resolution, but at the cost of the strainresolution. From FIG. 31, it can noted that, for very short gaugelengths, the spectral and spatial resolution seems to beFourier-transform limited, and an increase in gauge length (spatialresolution) leads to a decrease in RMS noise level (spectralresolution). However, for very large gauge lengths, a plateau is met,and the added value of a larger gauge length is minimal. Furthermore, itis obvious that, regardless of the gauge length, the noise level isalways much smaller with the ROGUE FBG than with the unexposed SMF-28fiber. Finally, in this example, a ROGUE FBG scanned with a 5.24 nmbandwidth required a gauge length of only 2 mm to beat the noise levelof SMF-28 with a 42.90 nm bandwidth and a 1 cm gauge length.

As can be understood, the examples described above and illustrated areintended to be exemplary only. Although the optical fibers are shownwith a circular cross-section, the optical fibers of other possibleembodiments of the optical fiber device can have any other suitableshape including, but not limited to, rectangular, hexagonal, and thelike. For instance, the DTSS system described herein can be adapted toperform optical time domain reflectometry (OTDR) in which a pulse issent and its return is monitored over time, and/or optical frequencydomain reflectometry (OFDR) in which each measurement includes a scan infrequency or wavelength. The scope is indicated by the appended claims.

What is claimed is:
 1. A method of fabricating an optical fiber device,the method comprising: positioning longitudinal portions of a pluralityof optical fibers alongside each other in a given geometricalrelationship, depositing liquid coating material around the longitudinalportions of the plurality of optical fibers; and the liquid coatingmaterial setting up around the longitudinal portions of the plurality ofoptical fibers thereby maintaining said given geometrical relationshipalong the longitudinal portions.
 2. The method of claim 1 wherein saiddepositing includes extruding the liquid coating material around thelongitudinal portions of the plurality of optical fibers whilemaintaining said given geometrical relationship.
 3. The method of claim2 wherein said extruding includes forcing the liquid coating material ata longitudinal position around the longitudinal portions of theplurality of optical fibers and moving the longitudinal portions of theplurality of optical fibers longitudinally during said forcing.
 4. Themethod of claim 3 wherein said moving includes longitudinally pulling onends of the plurality of optical fibers.
 5. The method of claim 3wherein said moving is performed as the longitudinal portions of theplurality of optical fibers are longitudinally received in an openinghaving an inner surface confining the plurality of optical fibers intothe given geometrical relationship.
 6. The method of claim 5 wherein theinner surface of the opening has a dimension below 1 mm, preferablybelow 500 μm and more preferably below 400 μm.
 7. The method of claim 1wherein said depositing includes maintaining the plurality of opticalfibers parallel to one another.
 8. The method of claim 1 wherein saiddepositing includes melting coating material thereby forming the liquidcoating material, and wherein said setting up includes cooling theliquid coating material.
 9. The method of claim 1 further comprising,prior to said depositing, heating a plurality of optical fiber preforms,and drawing the plurality of optical fiber preform into the plurality ofoptical fibers.
 10. The method of claim 9 further comprising, after saiddrawing, inscribing one or more optical gratings along a portion of eachof the plurality of optical fibers.
 11. The method of claim 10 whereinat least one of the optical gratings has a random continuousdistribution such that a return signal propagating in said optical fiberhas a full width at half maximum bandwidth ranging between about 0.1 THzand about 40 THz.
 12. The method of claim 1 wherein said positioninginclude positioning at least an additional component relative to theplurality of optical fibers, said depositing including depositing liquidcoating material also around said additional component, the liquidcoating material setting up around the additional component as well.